Architects and engineers that design HVAC systems for commercial buildings and other structures go to great lengths to ensure that those systems provide a consistent and reliable level of comfort to the occupants of those structures. HVAC designers carefully size the HVAC units to ensure delivery of the appropriate volume of conditioned air. Additionally, they design the ductwork to distribute the conditioned air to the various rooms and other areas of the structure at adequate rates and proportions. Furthermore, the designers select the spacing and configuration of the diffusers, registers, or terminals through which airflow is discharged (hereafter referred to generally as “diffusers”) to distribute and disperse the conditioned air into the rooms/areas so as to provide the desired level of comfort for the occupants.
Integral to this design is the need for the conditioned air to be dispersed from each diffuser at a volumetric flow rate that is at or within a predetermined range of a rate specified by the designer. Flow rates that deviate from those specified by the designers will result in room or area temperatures that deviate from the target temperature set at the controller/thermostat, which can compromise the comfort of the building occupants.
When new commercial HVAC systems are commissioned, the system requires balancing to ensure that the conditioned air is delivered from each diffuser at a volumetric flow rate that is at or within a range specified by the system designers. Balancing can also be required as a part of routine HVAC system maintenance or when the floor plan within a building is reconfigured.
Balancing a commercial HVAC system is not a trivial matter and requires the services of a qualified HVAC technician. Commercial HVAC duct runs can be complicated and can have many branches or zones, each of which has many diffusers, or nodes. Not only does each diffuser have its own damper for adjusting flow through that particular node, there are also dampers within the ductwork that can be used to control airflow to the various zones within the system. Once one considers that adjusting a damper for any zone or node will necessarily create a change in backpressure that affects the airflow through all other zones and nodes in the system, the complexity of the balancing task becomes clear.
Ceiling mounted diffusers of commercial HVAC systems are selected by the system designer from a finite number of configurations to diffuse and direct conditioned air into the building space in a predetermined pattern. While there are many different diffuser configurations from which to choose, a vast majority of the diffuser designs fall within or are based around a standard 24-inch by 24-inch footprint common to commercial drop ceiling tiles.
The National Environmental Balancing Board (“NEBB”) is an international certification association that, among other functions, certifies individuals and firms to commission, test, adjust, and balance HVAC systems. In addition to certifications, NEBB also provides equipment specifications and procedural standards. On the equipment side, one piece of equipment for which NEBB issues specifications is referred to a direct reading hood, which is used to measure air flow through a ceiling mounted diffuser. In this description, the more generic term “air flow hood” is used to describe a most commonly used form of a direct reading hood device. Those skilled in the art will appreciate that “direct reading hood” and “air flow hood,” as used in this description, are essentially interchangeable, i.e., the air flow hood described herein can be characterized as a direct reading hood within the NEBB specification.
Air flow hoods are instruments that are used by HVAC technicians to measure the airflow discharged through ceiling mounted diffusers of commercial HVAC systems. Air flow hoods are designed to be held in place over the diffuser. The hood acts as a duct that collects and redirects the air that is discharged from the diffuser. The air flow hood has the configuration of a converging-diverging nozzle with a throat through which the conditioned air is directed in order to measure its volumetric flow rate. Differential pressure is measured across an averaging pitot tube manometer located in the throat.
Averaging pitot tube manometers used in conventional air flow hoods typically include a plurality of tubes arranged in an array across the throat. The tubes define two channels (one for averaging upstream pressure and one for averaging downsteam pressure) that are fluidly connected to a single manometer. Ports spaced about the tubes face in upstream and downstream directions in the hood and are connected to the upstream and downstream channels, respectively. The airflow in the hood therefore creates a velocity pressure across the pitot tube array, with the high side total pressure being averaged by the upstream channel and the low side static pressure being averaged by the downstream channels. The intent is that, since the ports are spaced about the array, which extends across the cross section of the throat, the velocity pressure sensed via the array is an average velocity in the throat. This average velocity pressure can be used to calculate an average air velocity through the hood, from which a volumetric flow rate can be calculated.
Averaging pitot tube array manometers can be susceptible to errors. The differential pressures measured across the averaging pitot tube are very sensitive to variances in air velocity across the many ports, which can relieve pressure at some of the openings. This can be the case, for example, with highly non-uniform flow concentrations, where areas of relatively high or low concentrations happen to be located in the area of the pitot tube ports. In either case, the measured differential pressure will not produce an accurate airflow measurement.
Regardless of the particular configuration of the HVAC diffuser, the conventional air flow hood collects the discharged air and redirects the air toward the throat where the differential pressure measurements used to calculate the volumetric airflow through the air flow hood are taken. This collection and redirection, however, reduces the airflow rate through the hood, which creates backpressure in the HVAC system. As a result of this backpressure created by the air flow hood, airflow through the diffuser is reduced. Left unchecked, this will produce a corresponding error in the air flow hood airflow reading. Realizing that the differential pressures measured with a air flow hood can necessarily require a resolution of up to 0.001 inches of water column (IWC), these errors can become significant.
Additionally, conventional air flow hoods typically have an open cross section and the air directed through the hood is free to follow whatever flow path and pattern that physics dictates. Because of this, the bulk flow through the air flow hood is not uniform across the cross section of the hood, and the accuracy of the flow measurement can suffer. The redirection of flow in the hood can cause recirculation patterns in some regions of the hood, for example toward the middle regions of the hood, while the majority of the bulk flow is directed along other regions of the hood, such as along the sides. The conventional air flow hood thus can suffer from a lack of mixing, wherein the flow has a more blended, uniform flow pattern across the cross section of the hood.
Furthermore, the non-uniformity of the flow through the air flow hood will change depending on the configuration of the diffuser discharging the air. Because the averaging pitot tube manometers in the conventional air flow hood have fixed positions within the cross section of the hood, the accuracy of the net pressure measured can vary substantially with varying velocity profiles and the flow calculations resulting from these measurements, are unreliable and left to chance. In view of the above, it is readily apparent that the conventional air flow hood suffers from inaccuracies due to non-uniformity of flow and due to flow variances brought about by different diffuser configurations.